In the field of computer chip architecture, it is desirable to efficiently use system resources. One way to improve efficiency is to share resources, such as processing and memory resources, among various operations. In a system having multiple processors or multiple processes semaphore operations are used to maximize resources. A semaphore is a permission to use a system resource, such as a location in memory, that may be “locked” by a process performed by the computer system. By locking the semaphore, the process has access to the system resources so long as it retains control of the semaphore. When a process is done using a resource, it releases the semaphore so that another process may use the resource. The use of a semaphore ensures that other system processes will not try to access the same system resource while it is being used by another process.
A semaphore operation reads, modifies, and writes a particular memory location during a single atomic operation. Example semaphore operations include load and clear, fetch and increment, fetch and decrement, and compare and swap. These semaphore operations may be implemented in hardware, but circuitry for operations other than load and clear are complicated. One implementation of semaphore operations implements only a load and clear operation in hardware to capture the semaphore, and implements the other operations in software macros built around the load and clear operation. This system is used, for example, in precision architecture reduced instruction set computing (PA-RISC) architecture. A process running on the system then performs its operation using a sequence of instructions and writes the result to the semaphore's location to release the semaphore. The load and clear operation writes its value to zero indicating that the semaphore is in use.
A problem arises when the semaphore operations are interrupted by a system interrupt. A process may be interrupted at any time during its operation. This creates a problem when the process is interrupted while it has locked a semaphore. The process may stall or appear to “go to sleep” for a long period of time, tying up the semaphore in the process. A semaphore may be a significant resource bottleneck in the system, and the interrupt may cause significant problems for other processes that want to access a semaphore. Careful coding is one means of attempting to resolve the interrupt problem, but it requires substantial time and does not address unscheduled, asynchronous interrupts. Synchronous interrupts are those interrupts that are expected to occur at certain, scheduled times relative to code execution. External, or asynchronous, interrupts or traps, such as input/output (I/O) driver interrupts present greater problems. In some instances, it is possible for asynchronous interrupts to interrupt a semaphore operation and tie up system resources. What is needed is a better method and system for processing semaphore operations.